Abrasion-resistant magnetic recording tape



Patented Aug. 19, 1952 ABRASION-RESISTANT MAGNETIC RECORDING Ambrose F. .Schmclzle and. Esther E. Eastwold, St. Paul, Minn., ,assignors to Minnesota Mining. I & Manufacturingflompany, St.' Pau'l, Minn, a

corporation of Delaware N Drawing. ApplicatioriAprilZ,1949,

I Serial No. 85,266

This invention relates to surface-coated mag netic recording media, particularly in sheet or tape form, the surface coating being composed essentially of ferro-magnetic particles ina flexible binding medium comprising a blend of a high molecular weight vinyl polymer and a rubbery butadiene-acrylonitrile polymer.

In one popular method ofrecording signals on a medium such' as magnetic recording tape, the tape is pulled by a constant-speed capstan past a stationary magnetic head which forms apart of a recording circuit, where it is subjected to a fluctuating magnetic field,'the fluctuations being set up by the incoming signal. A magnetic record of the signals is thus impressed on the tape. When the tape is. subsequently pulled past a similar magnetic head connected in a reproducing circuit, the original signal is regenerated in that circuit.

Except for the brief period during which it is pulled past the magnetic heads, the tape is kept in the form of a more or less tightly wound roll. One of the requirements of a recording medium, particularly in the home entertainment field, is that the recording be permanent over long periods of time. Hence the rolls of tape are required to be capable of remaining in storage for years Without loss of playability. There must be no sticking together of the adjacent convolutions of the roll, nor any embrittlement of the binder such as might result in cracking or flaking of active material.

Loss of active material is disastrous, in the case of magnetic recording tape, for a number of reasons. The removal of relatively large sections of the active material by flaking causes complete loss of signal, either before or after recording. Greater or lesser amounts of the active material may be lost from an inferior tape product either by dusting from'a poorly adherent binder or by rub-off of traces of a soft binder and associated active material, usually at the area of contact between tape and magnetic head. Loss of material in whatever-form is indicative of wear, and under such conditions the useful life of the, tape is decreased. A more serious eflect is the building up of a deposit of the loosened material on the magnetic head, resulting in distortion of the signal and in lowered efficiency of recording and reproducing.

A further and obvious requirement of a satisfactory magnetic recording tape is that it have suitable magnetic properties. Magnetic iron oxide is commonly employed as the active material, and the powdered oxide is treated to secure 3 Claims. (01. i -76) certain desired values of coercive force, remanence, etc., before incorporation in thetape structure. It is necessary to retainv these properties while stillobtaininguniiorm distribution of the oxide particles, since uneven f distribution con.- tributes tofsuch undesirable phenomena as high noise level.-.-

Still further requirements of magnetic recording tape, particularly in the fields of highfidelity reproduction and accurate timing operations, are concerned with the dimensional stability of the product. The problem is rendered more acute by the accompanying requirement of extreme thinness or low space factor. a

The conventional binders employed, prior to the present invention, in making coated magnetic recordingltapes were either inherently defective in one or more'particulars, or contributed to deficiencies in other components of the product. Cellulosic. derivatives such as cellulose nitrate, for example, are necessarily plasticized with materials such as dibutyl phthalate in order to prevent brittleness and flaking of the coating. Aging. of such, a plasticized'coating results in loss of plasticizer by 'volatilization, and embrittlement of the remaining coating. Where these plasticized binders are applied to certain types of backings, theplasticizer is found, under certain test conditions, to slowly migrate into the backing, thus embrittling the coating and at the same time softening the backing and rendering it more easily stretchable. A further problem, where cellulose nitrate is employed, is the known tendency of the cellulose nitrate it self to deteriorate on prolonged storage, particularly at elevated temperatures.

A primary object of this invention is the provision of surface-coated magnetic recording media either in the form of cylinders, discs, sheets or the like or more particularly in the form of a thin, narrow, continuous tape which may be wound on reels and rewound for high-fidelity recording and reproducing of signals, and which avoids the above-noted as well as other deficiencies of the prior art. The desired product is non-blocking, i. e. may be unwound from roll form without sticking or jerking, and is dimensiona1ly stable. The tape product has sufiicient mechanical strength, toughness andfiexibility to be used on conventional magnetic tape recorder equipment without breakage. The magnetic coating is firmly bonded, flexible, :and of the correct hardness and toughness to remain as a smooth and continuous filmunder use conditions,

These and other objects and advantages are achieved in the novel constructions of the present invention, as exemplified by the following specific structures.

Exampl 1 tially reacted condensation product of one-mole of phenol, 0.25 mol of acetaldehyde and 9 5;mol of formaldehyde is spirit-soluble, i. e. soluble in lower alcohols, esters and ketonesand is suitable. An.eid enie .v h n, e i i= v o t m ri l ,PTO- vi des a strongly adherent bond between the .aee s efilmbas eou mesn i .e eting- "'[I he magnetic coating consists of two parts" by weight'ofa magneticrediron oxide powder uni.- formly dispersed in, and bonded with, a'high molegular weight vinyl resin polymer WhiChf haSJbQGH p ast iz dand d e wit sse p opo i o a com ati r bbe b t e-eor on tn nol mer- ."Ih a ineise p i vw bhid e ido a volatile.o sanid l ent. which s the vol iz a drehiored,leavinsa n o imat 4? g a ns of dry inspe 1 f sgin "Th e atine sfinahy s enasmoothlsur aee fini bypas in t e tap twice throusha sunerea en e .lnthisexam q the n rpol me is a n po ymeno ifiopar 'o nxLchMidemd 0 p rt o in a e a auand it. .e rubber Polymer is e eonolymer-,.made o 1 parts o u adieneelfi an 135. .parts .o .aerylonitr le .E n parts ,by

wei ht'ofthe rinylnolxmen toeet er-with a mal amoun of a lor an c l ad. s bi izer. and. one pa nt .of t e iresh ni. l d ru ber no merha dissolved ndzblende in m hylisobnt l e on .Br e linsof heru be y p l m r on ambhersr l i st pri r to dis olyi e -is found oi advanta eous. mine-cea in theret o sohdion and in reduc n the flnalv se s t o this p e- -.l ns hah hinder. o ut on. having a so ids co ent oi 30%,-is. t en eedded. the ma n tic. ir n ox d andwan a d t onal .ememti yof meth l .is l nty l e one. ,toR QV de .oomnosi ion havin a s id co tent. of 8 'I ismix u eis. mi ed Lina bal m ll ntilthe oxide isro mp t lylan u formly dispersed s smallnniiorm siz cl pa ti l s. whe Yi we rnn errtheln er sco e and th lmixtu when hea ed. and driedt n a. th i i n fee s ne Theindividualparticles of-the--pa-rticular magv neticpxide employed in this example w ere -of a blocky or chunky character-asseen under a microscope.

Coated tapeprepared with eitherof the-abovedescribed-dispersions was found to have excellent magnetic-and physical pmpertiesas demonstrated-by actual-usetests. The-tape-unwo und from the reel without sticking, and did not cause build-up of magnetic material on the magnetic head. Equally good results were obtained after prolonged storage at 120 F. as were obtained with the freshly prepared tape. Addi- ;,tionally, the tape was tested successfully at low temperatures, under which conditions nitrocellulose-bonded tapes are known to suffer se- '-vere cracking and flaking.

Qther. solvents or solvent mixtures may be used. "For example, the vinyl polymer may be dissolved in a mixture of methyl ethyl ketone -,and-toluene,and the rubbery polymer dissolved gin toluene;f;;the .two solutions are then blended .and the ma netic powder added in the ball mill. While not essential, small amounts of various I ajdditiyes may beincorporated in the tape in variousways for special purposes.

The organic lead compounds or other stabilizers for the vinyl polymer are included within this category, as are he tabilizers, a t ox dants or preservativ s. t n or isper inea ents. etc Whenet hiene i em loyed a aoom o ent lit e. s lvent. mixt re.- it is ound des rabl toadddispersin a ents such as-"A ,os 1O dioet lestero sodium s lf sue' i a Yelkin .le' ithini il, mixture, etc., in order to speed upthe ;disner sioniof the Pi m n the ball m l andp eyentsubsequ s t ing -.duing st a e in so ut on fo m-r 7 Example; 2

.11 thi ex mp .th prime-coated -.ee u1 tat r .o..m.r ..Examp .lwasiu ther coatedwit rains per 2.5 sqof adispe ion o.f tw part oi ane ioul ema etic ironroxi powder 'ingne part ,of afidzj dbl n o h v yl-polyme and .rhbbe po yme -identifie in Example. 1. T e. ,s.ome.what.- h gher. propo t on. f rub ery polymer was found to be required in,or,de r--to moyide adequat ees hi tybfi t mae et oeoa ins. w e h .ao on a t p xid manh les wassub titut d or. th l hunky t pe- .Th s t p produetelik ethat .of the. previous exampl :d not. .orae or .flake, did no lloseeae v a r al .b brasion..s ainst .the ma net c; head, andndid not-cause sti kin i th e 1 B sof t e: tap s were d mensionally-s able.under thetension en- ,eountered ,in norm l u e of th p odu t.- ven thou h their l e was J ssv than hat ,of many .prior,art. .tapeconstruc Example 3 ;Th in rope-fibertissue; paper (fliFlexropeW ;having,,an ,initial; caliper of 0500.175 calend neda aliper o -..:0- 91.-

a the m aturat d. wit a; gniented .zp dsticized o yme The im re at :sheet was sea ed-d rec y wi h t s sp nsion f ma neti oxideasd s hed T n 'rE amp e 2, and thee ated surface .supercalendered until smooth.-

- The impre nati s lu i rfor, the pap r-hash in was made y m xi to et 1 r .-:o. .410. vinyl .chloride yinyl .acetate copolymer, .1 .-.1 .p.ar sof..di ty1:..pht a a an 3 pa s-ref biliger (e. g., ,fV-l-N.stabilizer h forming them x teintoa. ho o neous she arru be m l addi .1.0 ..par.ts of T 02 p ment o the i h d is rsi the mi ed stoekzindlzrnar s of m t yl .iso ut l .ketone. About /2 ere-in z weight .ner 2A. sq- -i of, tissue was re for saturation. 7 This coatin "acts, as. ,a-priming ag nt qfor pr din .afirm bond .tw en.;th a k g t a netic ho tin a d. also adds t th st en f t paper backin t ou cb t d e e %9lylQI lfile polyme for magnetic particles-in ,coatedmagnetic re cording tapes. Monomericliquid plasticizers have generally been included in these prior art compositions, even where the application involved 1 far lower flexibility requirements than are involved in the use of these tapes. Additionally in; ;view of. the high pigment volume-loading required for desiredmagnetic properties, it would normallybe supposedthat considerableproportions of liquid type-plasticizers wovuld therefore berequired with the polymeric binders inproviding adequate flexibility in the magnetic coatings. Vinyl polymer binders containing liquid plasticizers would be subject to the same dee' ficiencies. as the plasticized nitrocellulose binders. Since in addition the vinyl polymers are in general more expensive than nitrocellulose, it is, not

- surprising that no attempt was made, prior, to

our invention, to utilize these materials as binders I for coated magnetic recording tapes.

Surprisingly, we have now found that oombir, nations of vinyl polymers and butadiene-acrylm. nitrile polymersas herein described, and without the addition of monomeric or liquid type plas ticizers, are fully adequate in flexibility for use as binders for the magnetic particlesof coated magnetic recording tape. In addition, these novel binder compositions provide improved re,-

sults in terms of permanence, heat-resistance,

performance at low temperatures, and, even more significantly, in terms of abrasion resistance. f I

This last-named property we have found may be measured, in terms which correlate well with results obtained in actual long-time use on a magnetic recording machine, by means of an abrasion resistance tester such as the Taber Abraser. In this instrument a 4-inch disc of the-coated sheet is'firmly held on a fiat hardrubber-surfaced turntable and rotated beneath two 2" x "-abrasive wheels revolving about axles parallel to the sheet and with their edge surfaces pressed against the coated surface. The wheels abrade a path in'the coating which is 3% inches outside diameter. [The numbenof revolutions of the disc requiredjor the abrasive wheels towear through the coating is takenias In the a measure of the abrasion resistance. test here described, abrasive wheel's consisting of abrasive particles in a rubbery binder and designated by the manufacturer as Calibrase CSlO wheels were used under a load of 1000 grams each. The test was run at room temperature. The thickness of magnetic coating was at least about 0.6 mil and not more than 1.0 mil, and results are given in revolutions per mil of thickness.

It was found that coated magnetic recording media having the conventional type of nitrocellulose-base binders failed in the abrasion test at 100-150 revolutions per mil. On the other hand, the coated sheet material of this invention, having equivalent proportional amounts of similar magnetic particles but employing the novel vinyl polymer butadiene acrylonitrile polymer binder here described, as more specifically set forth in the examples, provided an abrasion resistance value of at least about 300-500 revolutions per mil, and in some cases as high as 800 revolutions per mil.

We have also observed that binders having a Taber abrasion value as hereinabove defined of less than about 250-300 revolutions per mil show a decided lossof active material on extended use of the magnetic recording tape, whereas. above this value no detectable wear is observed under the sameconditions.

The invention has hereinabove been described in terms of specific embodiments. While certain equivalent materials and structures as herein noted are also contemplated, it is nonetheless true that the invention is quite specific .in the type of binder employed and in the relationship of the binder to the other components of the tape product.

With regard to polymers of vinyl chloride and vinyl acetate as employed in the specific examples, we have found that these polymers must 4 have a vinyl chloride content of at least about 86% and not more than about 91%, must bev soluble in methyl isobutyl ketone, and must have a molecular weight of at least about 10,000, in order to achieve proper balance of abrasion. resistance, heatresistance, toughness, flexibility, and coating properties. The remainder of the polymer (to make may be copolymerized vinyl acetate, or a mixture of vinyl acetate and vinyl alcohol, or a mixture of vinyl acetate and some other copolymerizable monomer. Mixtures of polymers may be used to advantage in many cases; for example, one-half of the vinyl chloride-vinyl acetate copolymer of Example 1 may be replaced by an equal weight of a copolymer of 91 parts vinyl chloride, 3 parts vinyl acetate and 6 parts vinyl alcohol, Other equivalent high molecular weight ketone-soluble vinyl polymers may'replace the vinyl chloride-vinyl acetate 00- polymer in whole or in part.

The butadiene-acrylonitrile polymer must be compatible with the vinyl polymer and provide the required degree of softness and flexibility while still maintaining a high degree of abra sion resistance. Combinations of butadiene and 25-45 parts of acrylonitrile.

The ratio between the two polymersilikewise is important, although depending to aQ'considerable degree onthe type and amount of magnetic oxide pigment; added, Theproportions employed inthe examples represent the approximate com-' positional limits of the specific binders there given, where iron oxide pigment represents about I of the total weight of binder and pigment. As high as four parts of magnetic iron oxide have in some cases been used with one part of' binder to produce coatings which are still adequately abrasion-resistant, but it is preferred to hold the proportions of oxide and binder between about 2:1 and about 3:1. Where much more than this proportion of oxide is added, the binder is insufiicient to hold the. pigment in place; with less oxide, magnetic efliciency is reduced.

While other prime-coats may be employed to provide a suitable bond between magnetic layer and backing layer, the particular mixture described has given excellent results and is preferred. Similarly, while thin paper as in Example 3, or other fibrous backings, or various low-stretch non-fibrous films, or films containing reinforcing fibers may be substituted for the cellulose acetate film of Examples 1 and 2, that particular film in conjunction with the specific primer coating: as described provides an entirelysatisfactory base for 1 our novel magnetic coatingvand is preferred by us.

There has thus been described a newpro-duct; particularly inthe formofmagneticrecordinga o tape: or sheet, comprising a thin, -fiex-ible back-' inganda firmly adherently attachedpermanent- 1 1y-- flexible and non-cracking 'magneticcoating As=-previously noted, the magnetic I coating: de- 1 sirably contains approximately two parts ofmag-' I m netic-iron "oxide particles uniformly dispersed and bondedwith approximatelyone=part of-a flexible and abrasion-resistant bindercompris ing a: high molecular weight; ketone-soluble vinyl p'oly mer andarubbery, 'ketone-soluble' ls butadiene-acrylonitrilepolymer; Within these compositional limitations, particularly: desirable results are obtained where the-binderconsists predominantly ofabout 60 80- part's of a -ketone= soluble -vinyl chloride polymer and "correspondingly'about 40-20 parts of a copolymer of about 75-155 parts of butadiene and about -45 *parts'-= ofv acrylonitrile, and wherein theuketone -soluble vinyl chloride polymer represents/a vinyl: chloride content 'of at least about 86%;- 25

With this particular class of 'binder'composr y tions;:- there is' obtaineda surprising combination 'of properties not heretoforetavailablei; In terms of-the properties to be obtained withthe coated .productin tapeiorm, the-material does not= block .(bec0me: tackyy at moderately: h ele-" va-tedtemperatures .norcrack or: -flake- (become britt1e)-at low :temperaturesw Posts made on the coated surface of "the: sheet show" th'ema-- terial to have the surprisingly high- Taber' .abrasion resistance value askherein identified of-atleast about 300 revolutions-pernnli Whatwe claim-is: r

1 As i ra -new product, asmagnetic n recording:

2.'-' As a now product, a- -magnetic recording-'-" tape, non-blocking at moderately elevated 'tem peratures and non-cracking at low temperatures? comprising a thin flexible non-magnetic' nonfibrous film backing and a' firmly "adherentlyat tached'magnetic coating, said coatingj being "a='- uniform dispersion of about" two*-parts'-of magnetic iron oxide pigment in one part of a binder:- and; said binder consisting of a-"blend ofabout 60'80 parts"of a--copo1ymer of about 86-91 parts of vinyl chloride and -"abo'ut*14-9 'parts of vinyl acetate and correspondingly about '4O- 2'Oparts or i a copolymer of about 55 75 fparts of "butadiene and about 45-25 parts of acrylonitri1e;-'the Taber abrasion resistance -va-lue of1the'-sai d- *product as herein-identified beinggat leas-t" about 300 revolutions per mm:

3.'As anew product; a magnetic recording? tape, non-blocking atmoderately elevated tom peratures and non 'cracking at 'lowtemperatures," comprising a V thin fiexible non-magnetic cellulose acetate film-and a magnetic coating firmlyad herently bonded thereto by a prime-coating of a blend of ;a rubberybutadiene-acrylonitfile" polymer andwa compatible spirit-.soluble'heaifreactive phenol-aldehyde .resin, said' magn'etic'. coating being; a uniform dispersion'rof 'about'twoi parts of blocky. magnetic iron oxide .pigment' in 7 one part of a bindenand'said binder consisting; of a blend of about 80"parts of'a copolymerlof" about 90 parts offvinyl chloride and aboutflO parts ofvinyl. acetate and correspondingly about. 20' partsof, a copolymer of about. 65part's 'oif butadiene. and about 35" parts 0f.j acry1onitrile, the Taber abrasion resistance value of the said; product as. hereinidentified being at least about 300rrevolutions perl AMBROSE. iFlj SCHMELZIIEIZ, ESTHER] E. IEAIS157570131111 medium in sheetortape:form; non-blocking at 'smoderately elevated I temperatures--- and; non cracking atlow temperaturescomprising-a fthln flexible non-magnetic backingand a firmlyad- I herently attachedflmagnetic coating; said coat-- ingtbeingx a uniform dispersion of about twoparts of magnetic iron oxidepigment in one] part of a binder, andsaidbinde'r comprising a blend ofzabout 60-.80 parts of amethyl isobutyl ketone-soluble."copolymer of. monomers comprisingwinyl chloride and vinyl. acetate: the :amountofivinyl chloride being about 86 91'percentof the-= total monomers, and correspondingly about 40 20 partsiofuatcopolymer of :about 55475 partsnof butadiene rand rabout'45t25' parts: of::acrylonitrile; the aTaber: abrasion resistance value. of the said:: product ashereintidentified being at le'ast raboutx 30(hrevolutionsper mil;

REFERENCES iCITEDfJ The following: referencesare r of .record-in'thei-i file" --of this patent:

UNITED STATES PATENTS? 

3. AS A NEW PRODUCT, A MAGNETIC RECORDING TAPE, NON-BLOCKING AT MODERATELY ELEVATED TEMPERATURES AND NON-CRACKING AT LOW TEMPERATURES, COMPRISING A THIN FLEXIBLE NON-MAGNETIC CELLULOSE ACETATE FILM AND A MAGNETIC COATING FIRMLY ADHERENTLY BONDED THERETO BY A PRIME-COATING OF A BLEND OF A RUBBERY BUTADIENE-ACRYLONITRILE POLYMER AND A COMPATIBLE SPIRIT-SOLUBLE HEATREACTIVE PHENOL-ALDEHYDE RESIN, SAID MAGNETIC COATING BEING A UNIFORM DISPERSION OF ABOUT TWO PARTS OF BLOCKY MAGNETIC IRON OXIDE PIGMENT IN ONE PART OF A BINDER, AND SAID BINDER CONSISTING OF A BLEND OF ABOUT 80 PARTS OF A COPOLYMER OF ABOUT 90 PARTS OF VINYL CHORIDE AND ABOUT 10 PARTS OF VINYL ACETATE AND CORRESPONDINGLY ABOUT 20 PARTS OF A COPOLYMER OF ABOUT 65 PARTS OF BUTADIENE AND ABOUT 35 PARTS OF ACRYLONITRILE, THE TABER ABRASION RESISTANCE VALUE OF THE SAID PRODUCT AS HEREIN INDENTIFIED BEING AT LEAST ABOUT 300 REVOLUTIONS PER MIL. 